Dear student,
I have given a simple explanation below. I hope you will understand.
Elimination reaction, any of a class of organicchemical reactions in which a pair of atoms or groups of atoms are removed from a molecule, usually through the action of acids, bases, or metals and, in some cases, by heating to a high temperature.
The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 reaction. The numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively. In rare cases, for molecules possessing particularly poor leaving groups, a third type of reaction, E1,exists.